The present invention relates to a position detecting device of the phase shift type which detects, as a change in the electrical phase angle of its output A.C. signal, a reluctance change generated by the movement of an object.
The present invention also relates to a time measuring device for measuring a time point at which a specific trigger pulse to be used in such a phase shift-type position detecting device is inputted thereto.
As a position detecting device utilizing a reluctance change, a rotation-type differential transformer called a "microsyn" is conventionally well known in the art. The microsyn transforms a detected rotational position into a voltage level and therefore has the disadvantage that it is susceptible to external disturbances and hence tends to make errors. For example, temperature changes may change coil resistance to create variations in detection signal level. Further, with such a microsyn, level attenuation amount in a signal transmission path extending from the detector to circuitry utilizing the detector's detection signals varies in accordance with the signal transmission distance or length, and level variations due to noise directly result in detection errors.
In view of the above-mentioned problems, there has been proposed a phase shift-type rotational position detecting device which is capable of accurately detecting a rotational position without being affected by its output level fluctuations due to external disturbances (for example, U.S. Pat. Nos. 4,604,575, 4,612,503 and 4,754,220). Also, there has been proposed a phase shift-type linear position detection device which is based on the same principle as the rotational position detecting device (for example, U.S. Pat. Nos. 4,556,886 and 4,717,874).
FIGS. 10 and 11 schematically illustrate the general structure of the proposed phase shift-type rotational position detecting device. In FIG. 10, a sensor section of the rotational position detecting device is shown. The sensor section generally comprises a stator 11a that has a plurality of poles A-D projecting perpendicularly from the rotation axis and also circumferentially spaced from each other by a predetermined interval or angle (90.degree.), and a rotor 11b provided within a stator space sordid by the poles A-D. That is, the stator 11a is provided in opposed relation to the outer surface of the rotor 11b.
The rotor 11b is made of such a material and in such a shape that a change in its rotational angle causes the reluctances of the poles A-D to be changed. In this illustrated example, the rotor 11b is in the form of a cyrinder that is eccentric with respect to the rotation axis. Primary coils 1A-1D and secondary coils 2A-2D are wound around the poles A-D of the stator 11a, respectively. In addition, other coils are wound around the first opposed pair of the poles A, C and the second opposed pair of the poles B, D in such a manner the two pairs may operate in a differential fashion and differential reluctance changes may occur.
The primary coils 1A and 1C wound around the first pair of the poles A and C are excited by sine wave signal sin.omega.t, while the primary coils 1B and 1D wound around the second pair of the poles B and D are excited by cosine wave signal cos.omega.t, so that composite output signal Y may be obtained from the secondary coils 2A-2D. As best shown in FIG. 12, the composite output signal Y is a signal Y=sin (.omega.t-.theta.) that is phase-shifted from first A.C. signal, i.e., reference signal (exciting signal for the primary coils) sin.omega.t, by an electrical phase angle corresponding to a rotational angle .theta. of the rotor 11b.
In the case where the induction-type, phase shift-type position detecting device as mentioned above is employed, position sensor unit will be needed which comprises a reference signal generating section that generates the first A.C. signal sin.omega.t or cos.omega.t, and a phase difference detection section that measures the electrical phase difference .theta. of the composite output signal Y to calculate position data indicative of the current position of the rotor 11b.
FIG. 11 illustrates an example of the structure of such a position sensor unit employed in the rotational position detecting device. As shown, the position sensor unit comprises a reference signal generating section that generates reference signals sin.omega.t or cos.omega.t, and a phase difference detecting section that detects the phase difference .theta. between the composite output signal Y=sin (.omega.t-.theta.) and the reference A.C. signal sin.omega.t.
In order to detect the phase difference .theta., the phase difference detecting section measures a time difference between the reference A.C. signal sin.omega.t and the composite output signal Y=sin (.omega.t-.theta.), because the phase difference .theta. is a value obtained by multiplying 2.pi. radian with a rate of a difference between the reference A.C. signal and composite signal Y to one period Tc of the reference A.C. signal. Accordingly, the phase difference detecting section measures, as a count Ny of a synchronous counter, a time difference between the reference A.C. signal sin.omega.t and the composite output signal Y and outputs it as the phase difference .theta..
The reference signal generating section generally comprises a clock oscillator 12, a synchronous counter 13, ROM's 14a, 14b, digital-to-analog (D-A) converters 15a, 15b and amplifiers 16a, 16b. The phase difference detecting section comprises an amplifier 17, a zero cross circuit 18 and a latch circuit 19.
The clock oscillator 12 generates reference clock signal CLx at a rapid and accurate frequency, on the basis of which other circuits operate. It is assumed here that the reference clock signal CLx has a frequency fx of 40.96 MHz and a period Tx of about 24.4 ns.
The synchronous counter 13 is a ring or cyclic counter that counts the reference clock signal CLx of the clock oscillator 12 and outputs its count Ny as an address signal to the ROM's 14a, 14b and the latch circuit 19 of the phase difference detecting section. The oscillation frequency fx of the reference clock signal CLx and the maximum cyclic count Nx of the synchronous counter 13 determine the frequency of the reference A.C. signal sin.omega.t, namely, primary carrier frequency fc.
If, for example, the oscillation frequency fx of the reference clock signal CLx is 40.96 MHz and the maximum cyclic count Nx of the synchronous counter 13 is 4,096, the primary carrier frequency fc will be 10 kHz (=40.96 MHz.div.4,096), and further if the maximum cyclic count Nx is 8,192, the primary carrier frequency fc will be 5 kHz (=40.96 MHz.div.8,192). FIG. 12 illustrates the relationships among the reference A.C. signal sin.omega.t, composite output signal Y=sin (.omega.t-.theta.) and reference clock signal CLx in the case where the maximum cyclic count Nx is 8,192.
The ROM's 14a, 14b store therein amplitude data corresponding to the reference A.C. signals sin.omega.t and cos.omega.t and generate amplitude data of the reference A.C. signals sin.omega.t or cos.omega.t in response to an address signal (count Ny) provided from the synchronous counter 13. More specifically, the ROM 14a stores amplitude data of the reference A.C. signal cos.omega.t, while the ROM 14b stores amplitude data of the reference A.C. signal sin.omega.t. Accordingly, when the ROM's 14a, 14b are provided with the same address, two kinds of reference A.C. signals sin.omega.t and cos.omega.t are outputted from the ROM's 14a, 14b. It is to be appreciated that two kinds of reference A.C. signals sin.omega.t and cos.omega.t can also be obtained by reading only one of the ROM's 14a, 14b with address signals having different phases.
The D-A converters 15a, 15b convert digital amplitude data read out from the ROM's 14a, 14b into analog signals which are then fed to the respective amplifiers 16a, 16b. The amplifiers 16a, 16b amplify the analog signals fed from the D-A converters 15a, 15b and apply the thus amplified signals as reference A.C. signals sin.omega. t and cos.omega.t to the primary coils 1A, 1C and 1B, 1D, respectively. When the count of the synchronous counter 13 is Nx, the count Nx corresponds to one period Tc (200 .mu.s) of the reference A.C. signal sin.omega. t, cos.omega.t, and phase angle change corresponding to a time of Tc/Nx (=24.4 ns) is needed for the synchronous counter 13 to count up by one. This means that the count Nx corresponds to the maximum phase angle, 2.pi. radian of the reference A.C. signal sin.omega.t, cos.omega.t, and one count-up of the counter 13 represents a change of rotational position by an angle corresponding to 2.pi./Nx radian.
The amplifier 17 amplifies the composite value of secondary voltages induced in the secondary coils 2A-2D and delivers the amplified composite value to the zero cross circuit 18.
The zero cross circuit 18 a zero cross point at which composite output signal (secondary voltage) Y=sin (.omega. t-.theta.) induced in the secondary coils 2A-2D of the rotational position detecting device changes from a negative voltage over to a positive voltage, so as to output a trigger pulse TGP to a flip-flop circuit 20.
The flip-flop circuit 20 receives at its clock pulse terminal C the reference clock signal CLx provided from the clock oscillator 12 and receives at its input terminal D the trigger pulse TGP provided from the zero cross circuit 18, so that it outputs a latch pulse LP in synchronization with the rising edge of the reference clock CLx after the trigger pulse TGP has been input. This latch pulse LP is such a pulse that rises when the counted value or count (hereinafter the terms count and counted value will be used for the same meaning) of the synchronous counter 13 is stabilized, i.e., in the middle of its count-up operation.
At a time point when the latch pulse LP has been input, the latch circuit 19 latches the counted value Ny of the synchronous counter 13 which has been initiated in response to reference clock signal fx at the rise of the reference A.C. signal. Accordingly, the counted value Ny as latched in the latch circuit 19 accurately represents the time difference, namely, phase difference between the reference A.C. signal and the composite output signal (composite secondary output).
In clearer terms, the composite output signal Y=sin (.omega. t-.theta.) is delivered to the zero cross circuit 18, which outputs a trigger pulse TGP to the flip-flop circuit 20 in synchronism with the timing when the amplitude of the composite output signal Y changes from a negative voltage to a positive voltage. The flip-flop circuit 20 gives the latch circuit 19 a latch pulse LP which is synchronous with the rise of a reference clock pulse CLx, upon which the latch circuit 19 latches the counted value Ny of the synchronous counter 13 in response to the rise of the latch pulse LP as shown in FIG. 12.
At that time, because one cyclic period of the synchronous counter 13 is coincident with one period of the sine wave signal sin.omega. t, the latch circuit 19 latches such a counted value Ny that corresponds to the time difference between the reference A.C. signal and the composite output signal Y=sin (.omega. t-.theta.). Therefore, the latch circuit 19 outputs the thus-latched counted value Ny as digital position data. Then, by multiplying this digital position data Ny by 2.pi./Nx, rotational position of the rotor 11b in its rotation direction can be calculated.
In this manner, the phase shift-type position detecting device employs a counter circuit to measure a time difference corresponding to a phase difference .theta. between the reference A.C. signal sin.omega. t and the composite output signal Y=sin(.omega. t-.theta.), and then provides the counted value of the counter circuit as rotational position data.
As mentioned above, the phase shift-type position detecting device latches a counted value of the synchronous counter 13 at a time when the flip-flop circuit 20 outputs a latch pulse LP after the zero cross circuit 18 has output a trigger pulse TGP, and then outputs the latched counted value as position data Ny.
Namely, if the oscillation frequency fx of the clock oscillator 12 shown in FIG. 12 is 40.96 MHz and the circulation counted value Nx of the synchronous counter 13 is 8,192, the phase shift-type position detecting device outputs, every 0.2 ms (200 .mu.s), position data having a detection accuracy (resolution) corresponding to one rotation (2.pi. radian) as divided by 8,192. Or, if the oscillation frequency fx of the clock oscillator 12 is 40.96 MHz and the circulation counted value Nx of the synchronous counter 13 is 4,096, the phase shift-type position detecting device outputs, every 0.1 ms (100 .mu.s), position data having a detection accuracy (resolution) corresponding to one rotation (2.pi. radian) as divided by 4,096.
Since, as mentioned earlier, the detection accuracy (resolution) of the position detecting device is a value dependent on the circulation count value Nx of the synchronous counter 13, detection accuracy can be improved by simply increasing the circulation count value Nx. But, in such a case, simply increasing the circulation count value Nx results in an increase in output period of the position data (decrease in primary carrier frequency fc), and hence the device can not be utilized in a realtime position control system etc. In order to increase the maximum count value Nx without the primary carrier frequency fc being decreased, it suffices only to increase the oscillation frequency fx of the reference clock signal CLx. However, if the oscillation frequency fx is set to be more than twice about 40 MHz, substantially no electronic components (counter circuit etc.) will operate accurately at such a high frequency, and hence as a matter of fact it is not feasible to increase the oscillation frequency fx of the reference clock signal CLx.
Therefore, with the position detecting device today, the oscillation frequency fx of the reference clock signal CLx is set at 40.96 MHz and the circulation count value Nx is set at 8,192 or 4,096, with the primary carrier frequency fc being set at 5 or 10 KHz.
As the result, the following problems are encountered when such a phase shift-type rotational position detecting device is mounted on the rotation shaft of a motor.
FIG. 13 illustrates a relationship between reference A.C. signal sin.omega. t and associated composite output signal Y employed in such a phase shift-type rotational position detecting device.
In response to angular movement of the motor, the phase difference .theta. between the reference A.C. signal sin .omega. t and the composite output signal Y gradually becomes greater as indicated by .theta., 2.theta., 3.theta. . . . . This phase difference .theta. corresponds to counted value Ny of the synchronous counter 13 that is sampled every 0.2 ms in synchronism with latch pulse LP, and it also represents a value dependent on the rotation speed of the motor. Thus, as the rotation speed of the motor (rotation shaft) increases, the phase difference .theta. becomes greater, while as the rotation speed decreases, the phase difference .theta. becomes smaller.
Referring to FIG. 13, position data DA4-DA0 represent the lower five bits of position data Ny output from the latch circuit 19 when the primary carrier frequency fc is 5 KHz and the rotation speed of the motor is 30 rpm, and position data DB4-DB0 represent the lower five bits of position data Ny output from the latch circuit 19 when the primary carrier frequency fc is 5 KHz and the rotation speed of the motor is 40 rpm.
If the primary carrier frequency fc is 5 KHz and the rotation speed of the motor is 30 rpm, rotational position data indicative of rotational movement for 0.2 ms take a value of 2.pi..times.30.div.(60.times.5,000)=.pi./5,000 radian, and rotational position data corresponding to one count of the synchronous counter 13 takes a value of .pi./8,192 (=.pi./4,096). Thus, in this case, the rotational position data indicative of rotational movement for 0.2 ms is smaller in value than the rotational position data corresponding to one count of the synchronous counter 13. In other words, time difference corresponding to the phase difference .theta. between reference A.C. signal sin.omega. t and composite output signal Y is smaller than the period Tx of the oscillation frequency fx.
Accordingly, even if the position detecting device outputs position data in response to latch pulse LP, the position data may have some unchanged portion. For example, for 0th and first latch pulses LP, position data DA4-DA0 are "00000"; for fifth and sixth latch pulses LP, position data DA4-DA0 are "00100"; for eleventh and twelfth latch pulses LP, position data DA4-DA0 are "01001"; and for sixteenth and seventeenth latch pulses LP, position data DA4-DA0 are "01101". Likewise, for both twenty-first and twenty-second latch pulses LP, and twenty-sixth and twenty-seventh latch pulses LP, position data DA4-DA0 are not changed.
Although the motor is actually caused to continuously rotate at a given speed (30 rpm in this case), there are produced changed and unchanged portions in the position data outputted every 0.2 ms. This is because the time difference corresponding to the phase difference .theta. between reference A.C. signal sin.omega. t and composite output signal Y is smaller than the period Tx of the oscillation frequency fx. Therefore, the position data apparently shows the motor is not in rotational movement, although the motor is actually caused to continuously rotate at a given speed (30 rpm in this case).
On the other hand, if the primary carrier frequency fc is 5 KHz and the rotation speed of the motor is 40 rpm, the rotational position data corresponding to one count of the counter 13 takes a value of .pi./4,096 radian, but the rotational position data indicative of rotational movement within 0.2 ms takes a value of 2.pi..times.40.div.(60.times.5,000)=.pi./3,750 radian that is greater than the value taken when the rotation speed is 30 rpm. That is to say, in the case where the rotation speed is 40 rpm, the rotational position data indicative of rotational movement within 0.2 ms is greater in value than the rotational position data corresponding to one count of the counter 13, and the time difference corresponding to the phase difference .theta. between reference A.C. signal sin.omega.t and composite output signal Y is greater than the period Tx of the oscillation frequency fx. Accordingly, the synchronous counter 13 counts up from the previous count at least once each time latch pulse LP is output. Thus, contrary to the above-mentioned case where the rotation speed is 30 rpm, position data Ny is output which changes its value in response to each latch pulse LP.
It should be understood that, in the case where the oscillation frequency fx of the clock oscillator 12 is 40.96 MHz and the primary carrier frequency fc is 5 KHz, rotational position data indicative of rotational movement within 0.2 ms is greater in value than the rotational position data corresponding to one count of the counter 13, 2.pi./8,192&gt;2.pi..times.Nr.div.(60.times.5,000) and hence the rotation speed Nr&lt;60.times.5,000.div.8,192.apprxeq.36.6 rpm.
Similarly, in the case where the oscillation frequency fx of the clock oscillator 12 is 40.96 MHz and the primary carrier frequency fc is 10 KHz, the phenomenon occurs when the rotational position data indicative of rotational movement within 0.1 ms is greater in value than the rotational position data corresponding to one count of the counter 13, 2.pi./4,096&gt;2.pi..times.Nr.div.(60.times.10,000) and hence the rotation speed Nr&lt;60.times.10,000.div.4,096.apprxeq.146.5 rpm.
As the result, when there is produced any unchanged portion in the position data output in response to each latch pulse LP despite the fact that the motor is continuously rotating at a given speed, it is observed from the position data as if the motor rotation speed is fluctuating, which becomes a significant problem in carrying out control of the rotation speed of the motor or positioning control.
It should now be appreciated that the clock oscillator 12, synchronous counter 13, latch circuit 19 and flip-flop circuit 20 shown in FIG. 11 together constitute a time measuring device which counts reference clock pulses generated by a crystal oscillator or the like and thereby measures a output time point at which trigger pulse TGP is output.
More specifically, the prior art time measuring device counts reference clock pulses having a frequency fx of about 40 MHz which are generated by crystal oscillator or the like, detects a count value corresponding to a time to be measured (in this case, output timing of the trigger pulse TGP), and measures the time on the basis of the counted value (i.e., by multiplying the counted value by one period of the reference clock signal).
However, the minimum unit time detectable by the device is dependent on the oscillation frequency fx of the reference clock signal CLx. For example, if the oscillation frequency fx is 40.96 MHz, one period of the reference clock signal is about 24.4 ns. Therefore, by doubling the oscillation frequency fx, for example, said one period, namely, minimum measurable time unit can be halved for higher resolution. But, when the oscillation frequency is increased more than twice as high as about 40 MHz, few electronic components (counter circuit etc.) can accurately operate at that frequency, and so it is impossible or difficult to achieve a higher resolution measurement.